
390 THE AMERICAN NATURALIST. [VoL. XXXVI. 
in the typical arthropod position. In many cases most of the 
body was probably concealed, leaving only the prominent median 
eyes exposed. The presence of paired oarlike appendages indi- 
cates the power of free swimming, but the more or less rigid and 
clumsy appendages and heavily armored body could have pro- 
duced little more than brief, spasmodic excursions, like those 
of adult Limuli and eurypterids, or jerky, intermittent flights 
through the water, like those of a copepod. And, just as in 
these examples the shape of the body and the position of the 
appendages in reference to the center of gravity compel the 
free-swimming individual to reverse the usual position of dorsal 
and ventral surfaces, so in the Peltacephalata the prevalence of 
the same conditions must have forced them, after leaving the 
bottom, to turn over and swim with the neural side uppermost, 
in the true vertebrate position. The swimming movements 
were probably aided in some cases by numerous small appendages 
on the head and trunk.  Fishlike caudal fins and tail were 
used in swimming and in reversing the position of the dorsal 
and ventral surfaces. 
It was not till this new method of locomotion had completely 
replaced the old that the eyes left the hamal surface (their 
position in most adult arthropods) and returned to the neural 
surface of the body (their position in embryo arthropods and 
their permanent position in vertebrates). 
The exoskeleton was a true dermal armor of ectodermic origin, 
intermediate between the type presented by Limulus and that 
of the more modern vertebrates. It consisted of three prin- 
cipal layers, the middle one containing large, more or less 
regular spaces or cancella. The matrix was strongly laminated 
and penetrated by numerous dentine-like tubules, or pore canals, 
and contained either unipolar or multipolar osseous lacuna. 
The trunk was covered with rhomboidal scales or with seg- 
mentally arranged ringlike plates. The presence of a system 
of superficial sense organs is indicated by numerous pitlike 
markings arranged in linear series. 
A flattened cartilaginous cranium was present, but notochord 
and vertebral arches were absent or rudimentary. Median and 
lateral eyes were enclosed in bony orbits, sometimes protected 




